This invention relates to a process of removing pollutants from exhaust gases by means of solids in a circulating fluidized bed which are circulated through a fluidized bed reactor, a separator, and a return line.
The flue gases produced by the combustion of fossil fuels may contain oxides of sulfur, particularly sulfur dioxide, in substantial proportions, which depend on the sulfur content of the starting materials. The exhaust gases produced by the continually increasing number of refuse incinerating plants also contain sulfur oxides and as a result of the combustion of plastic materials, which are virtually always present, contain hydrogen chloride and hydrogen fluoride as additional impurities. Pursuant to the regulations for the protection of the environment, such impurities must be removed from the exhaust gases before these are discharged into the atmosphere.
By far the largest number of processes of purifying exhaust gases comprise a wet scrubbing step for removing the above-mentioned impurities and use mainly solutions or suspensions of substances which react with the pollutants (Ullmann's Encyklopadie der Techn. Chemie, 3rd edition, Vol. 2/2, 1968, page 419).
Other processes comprise a dry scrubbing of the gases, wherein the gases are passed through a fixed bed of particulate solids, such as activated carbon or brown coal coke, which react with the impurities. Exhaust gases can also be purified by means of a so-called traveling bed, in which the solids are increasingly laden as they descend in the reactor and the solids are finally discharged. Fresh material at a rate corresponding to the discharge rate is fed to the upper portion of the reactor (Meier zu Kocker "Beurteilung und Aussichten von Verfahren zur Rauchgasentschwefelung"; V.G.B. Kraftwerkstechnik 53 (1973), pages 516 et seq.).
In another known process, sulfur oxides, e.g., are removed from gases in that absorbents are pneumatically introduced into the gases, the resulting gas-solid dispersion is pneumatically conveyed through a reaction zone and then through a residence zone, and the solids are subsequently separated from the gas. After certain regenerating steps, a partial stream of solids is finally recycled to the cleaning step (U.S. Pat. No. 3,485,014).
Another known system for a dry removal of pollutants from flue gases follows the combustion zone in the direction of flow of the flue gases in a region where the flue gases are at a temperature of 700.degree. to 900.degree. C. That system comprises a fluidized bed which occupies the entire cross-section of flow of the flue gases, and/or a circulating fluidized bed, and said fluidized bed is fed with an adsorbent consisting of calcium carbonate or magnesium carbonate (Laid-open German Application No. 3009366). The perforated bottom of the fluidized bed reactor is suitably cooled.
Wet scrubbing processes have the serious disadvantage that the formed sludge contains sulfites, sulfates and possibly also chlorides and fluorides, is difficult to discard and the purified exhaust gases must be reheated. The disadvantages of the known dry scrubbing processes using a fixed or traveling bed reside in that the absorbents are coarse-grained so that their capacity to bind the impurities contained in the exhaust gas is utilized only to a very low degree. Due to the high rate of gas to be purified and the relatively low permissible gas velocity, reactors of considerable size are required.
The important disadvantages of the process according to U.S. Pat. No. 3,485,014 reside in that the gas stream to be purified must be divided and the absorbent must be precisely proportioned in suitable equipment which must be wear-resistant. Another disadvantage is that the residence time of the gases in the reaction zone is too short for an adequate removal of the impurities thus requiring a reactor having a considerable overall height.
In the removal of pollutants from the flue gases at 700.degree. to 900.degree. C. in accordance with Laid-open German Application No. 3009366, difficulties are involved in that special measures and alterations of the furnace are required in order to adjust the flue gases to a temperature in the range of 700.degree. to 900.degree. C. because the flue gases are usually at a higher temperature as they leave the combustion chamber and at a lower temperature as they leave the waste heat boiler.
In connection with a special operation of removing hydrogen fluoride from exhaust gases, particularly from exhaust gases formed in the electrolytic production of aluminum, it is known to supply the hydrogen fluoride-containing gases as a fluidizing gas to a fluidized bed reactor and to control the velocity of the fluidizing gas in such a way that a circulating fluidized bed is formed which is circulated through the fluidized bed reactor, a succeeding cyclone separator and a return line (Laid-open German Application No. 2056096). It has been mentioned that the solids used to form the circulating fluidized bed may consist of alumina and/or sodium aluminate and are recycled to the electrolytic cell.
The above-mentioned process is not only used to purify specific exhaust gases but also has the object to recover the fluorine content of the exhaust gas in such a form that it can be recycled to the electrolytic cell so that less fluorine is required.
In the purification of exhaust gases, however, a recovery of sulfur oxides or other pollutants is usually not required but the laden sorbent is used, e.g., for construction purposes or as packing material in mines or may be discarded.